GUI: Connect Event label change to Diagram

The connection is managed manually
since QGraphicsItem doesn't inherit from QObject
for performance reasons.

gui/diagram.cpp

@@ -49,6 +49,13 @@ const double Event::m_labelBoxHeight = 4;
 Event::Event(model::Element *event, QGraphicsItem *parent)
     : QGraphicsItem(parent), m_event(event), m_typeGraphics(nullptr)
 {
+    m_labelConnection = QObject::connect(event, &model::Element::labelChanged,
+                                         [this] { update(); });
+}
+
+Event::~Event() noexcept
+{
+    QObject::disconnect(m_labelConnection);
 }
 
 QSize Event::units() const

gui/diagram.h

@@ -48,6 +48,8 @@ namespace diagram {
 class Event : public QGraphicsItem
 {
 public:
+    ~Event() noexcept override;
+
     QRectF boundingRect() const final;
     void paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
                QWidget *widget) final;
@@ -97,6 +99,7 @@ class Event : public QGraphicsItem
 
 private:
     QGraphicsItem *m_typeGraphics; ///< The graphics of the derived type.
+    QMetaObject::Connection m_labelConnection; ///< Tracks the label changes.
 };
 
 /**